Drina01 The story you write is very sad, and like others, I think it sounds as if your daughter is in a relationship involving coercive control - and about that you can do little, except be watchful and prepared to offer unconditional uncritical help if and when it is needed.
But I will make another comment, I think you have made yourself too available to your daughter and family. I fully understand that when our children start families, we want to give them all the help we can, but when we make them the centre of our lives, giving them everything they want whether babysitting or money and being willing at every turn to do what they want, we assume the response will be reciprocal from our child, but too often it leads to us being taken for granted, used = I might say, emotionally abused, by the child we have done so much to help.
In this case, I think your SiL is extending his control to ensnare you as well. I think it is time for you to draw back, to be less available when your daughter or SiL demand help, certainly do not support them anymore financially, even when they use the children's needs to emotionally blackmail you and, as your DH and DS are clearly supportive. develop a seperate independent life that helps you to cope with this problem.
All of us want to protect our adult children as we protected them when they were tiny, but often our interference makes things worse not better. All of us have at sometime or another seen a grown -up child in trouble and had to stand back and do nothing because getting involved will make the problem worse not better. Standing and watching is not easy.
But I think you need to start distancing, but be watchful.